Effect of humidity and hydrogen on the promotion of indentation crack growth in lead-free ferroelectric ceramics

The effect of humidity and hydrogen on the growth of an unloaded indentation crack in lead-free potassium sodium niobate (KNN) ferroelectric ceramics has been investigated. The results indicate that crack growth of an unloaded indentation occurs in humid air of 70 and 90% RH without the presence of an electric field or mechanical stresses but not in air with RH ≤ 30%. Growth of an indentation crack also occurs in dry air when the field is larger than the threshold field Eth(y) = 0.01Ec (normal to the poling direction) or Eth(z) = 0.05Ec (parallel to the poling direction), and the incubation time decreases for larger fields. The change in crack growth length in humid air under a sustained field, Δc, is composed of three parts; i.e., Δc = Δc1 + Δc2 + Δc12, where Δc1 is the incremental growth in humid air without a field, Δc2 is that in dry air under a sustained field and Δc12 is that induced by the coupling effect of the electric field and humidity since humidity promotes domain switching. Crack growth of an unloaded indentation can occur during hydrogen charging, and the threshold stress intensity factor of hydrogen-induced delayed cracking, KIH, as well as the fracture toughness of hydrogenated ceramics, KIC(H), decreases with increasing hydrogen concentration.